Pyrolytic carbon coating process



United States Patent ABSTRACT OF DISCLOSURE,

This invention relates to a process for preparing particles having ahigh density, isotropic pyrolytic carbon coating by contacting theparticles in a fluidized bed with a coating gas selected from the groupconsisting'of propane, 1,3 butadiene and propylene at a temperaturebetween 1l001300 C.

cross references to related applications.

S.N. 538,923, filed on Mar. 29, 1966, in the vnames of Hans Beutler etal., for Method for Applying Low Density Carbon Coatings.

S.N. 546,490, filed on Apr. 28, 1966, in the names of Ronald L. Beattyet al., for Method for Preparing Actinide Oxide Fuel Particles .andArticle Thereof, and now Patent Number 3,301,763.

Background of the invention This invention described herein was made inthe course of, or under, a contract with the US. Atomic EnergyCommission. It relates generally to coated fuel particles and moreparticularl to a method for preparing fuel particles having a highdensity, isotropic pyrolytic carbon coating. v 7

Extensive investigation has been conducted on various types of coatingsfor nuclear fuel particles. It is generally agreed that pyrolytic carbonis ahighly suitable coating material and is preferred as a coating forcarbide and oxide fuel particles. Moreover, it has further been agreedthat in order to insure integrity of the fuel particle, multilayers ofcoating material should be employed. In one application it has beenfound that by providing a highly porous, low density carbon coatingfollowed by an outer impervious coating of high density pyrolyticcarbon, fuel failure under irradiation, due to fission product recoildamage and rupture of the coating from thermal stresses and internalpressure build-up from fission product gases, can be greatlyminimizedLThe low density coatings can be applied using acetylene gas attemperatures of about 1000 C. to 1200 C. A more complete description ofthis method for depositing a low density, porous carbon coating as aninitial coating may be found in copending application S.N. 538,923 filedon Mar. 29, 1966, in the names of Hans Beutler et al., for Method forApplying Low Density Carbon Coatings.

Experimentation with pyrolytic carbon coatings indicated that in orderto serve a an impervious outer coating it must be isotropic, i.e.,havethe same properties in all directions so that strains will not beinduced into the coating by thermal effects or b radiation damage, and

coating'at such high temperatures. Oxide fuel particles when coated,either bare or with a first highly porous, low density carbon coating,with pyrolytic carbon by de-' composition of methane at a temperatureabove 2000 C; undergo a deleterious reaction with the coating during thehigh temperature deposition. Previously, it has been necessary toprovide a thin, high density, sealer coating of carbon over the initialhighly porous, lowdensity carbon coating before making the final hightemperature deposition to prevent reaction between the oxide core andcoating. This method is disclosed in copending application if S.N.546,490 filed on Apr. 28, 1966, in the names of Ronald L. Beatty et al.,for Method for Preparing Ac-* tinide Oxide Fuel Particles and ArticleThereof. It has been shown that the release of fission products ofcoated particles during irradition depends to a large degree on the fuelcontamination level of the coating; It iswell known that fuel migrationduring the application of the coating is mainly responsible for thiscontamination. Fuel migration particularly in case of carbide fuels isstrongly temperature dependent and is insignificant below 1500 C.

' Therefore a low temperature coating process enables the application ofvery clean coatings resulting in low fission gas release rates. It istherefore highly desirable to provide a method for coating all types ofnuclear fuel particles with a high density, isotropic pyrolytic carboncoating at relatively low temperatures without resort to additionalcoating operations, as well as to efiect such coating operation withoutnecessitating a transfer of particles from a low temperature coatingfurnace to a high temperature furnace in a single coating run, which hasbeen required in the prior art method.

Summary of the invention coatings of high density, i.e., within therange of 1.95

have high density. While methane has been used to de-.

posit pyrolytic carbon at various temperatures ranging from 1300 C. toabove 2000 C., depositions at the lower temperatures resulted inanisotropic deposits; and only at coated with this high density,isotropic pyrolytic carbon to 2.1 gm./cc., could be obtained employingpropane, 1,3 butadiene or propylene as the coating gas within a criticaltemperature range which is markedly lower than those employed in priorart processes. Moreover, deposition is, rapid requiring only aboutone-fifth the time required in the prior art. Additionally, thetemperature drop within the reaction zone upon initiation of thedeposition operation was significantly lower than in prior art processeswhich beneficially afforded a high degree of control over the coatingoperation. The process is highly useful in the preparation of coatedparticles which are suitable for use in nuclear reactors.

Description It should be apparent that the present invention isapplicable to coating any of the nuclear fuels such as thorium carbide,thorium oxide, uranium oxide, carbide,

uranium nitride, plutonium carbide, plutonium oxide,

, iiuidizing gas with the deposition being carried out at an PatentedOct. 7, 1969 initial temperature of .1050" C. It will be apparent herethat the invention is not to be construed as limited to preparation ofmulti-layer carbon coated fuel particles but rather has application toany process for preparing fuel particles in which the outer layer is ahigh density, isotropic pyrolytic carbon coating whether it be as asingle or multiple layer coating.

After the low density, porous carbon coating is deposited to a desiredthickness such as about 50 microns, this coating phase is terminated bysubstitution of helium for the acetylene and the reactor temperatureequilibrated to a temperature within the range of 1110 to 1300" C. Thecoating and fl-uidizing gas comprising propane, 1,3 butadiene .orpropylene, in accordance with this invention, is then substituted forthe helium gas to initiate deposition, of the isotropic, high densitypyrolytic carbon coating. The temperature at which the deposition iscarried out is critical. As the temperature goes down (i.e., from about1250 C.) the deposition rate (and efficiency) of the coating process isreduced and the anisotropy factor is increased. Hence, when temperaturesbelow 1100.

C. are employed high densities of about 2.0 gm./cc. can be achieved, butwith an increase in anisotropy factors. At temperatures above 1300 C.the density of the pyrolytic carbon coatings drops 01f. Accordingly,temperatures between 1100-1300 C. are required for the preparation ofhigh density, isotropic pyrolytic carbon coatings.

The coating operation may be effected by providing the coating gaseither undiluted or as a mixture with an inert fluidizing gas, such ashelium. The supply rates for the coating gas are, preferably, between1-4 cm. /min.-cm. of particle surface area. Within this range of supplyrates the proportion of the coating gas, when diluted with helium, isnot critical, and suitable coating runs have been obtained employing atotal flow rate of 4 liters/ minute with ratios for diluted runs of 1:3to 3:1 coating gas to helium. As expected, the highest deposition rateswere obtained with undiluted coating gas at 4 liter's/ minute flow rate.

Where 1,3 butadiene or propylene is employed as the coating gas,additional advantages are afforded. Characteristically, thermal crackingof hydrocarbon gases which crack endothermically produces largetemperature drops (up to 300 C.) upon initiation of the coatingoperation. With propane, which cracks endothermically (24. 8kg.-cal./mole), the reactor zone temperature dropped about 150 C. uponinitiation of the coating op eration. To compensate for this, reactor isinitially heated to temperature which is slightly higher than that atwhich the coating is to be carried out. Where 1,3 butadiene or propyleneis employed, however, the temperature drop has been found to besignificantly lower (about 50 C.), thereby beneficially affording acloser control over the temperature at which the deposition isconducted. The advantages afforded by effecting a deposition at anessentially constant deposition temperature throughout the coatingoperation will be readily appreciated by those skilled in the art.

Having described the invention in a general fashion the followingexamples are given to indicate with greater particularity the processparameters and techniques. Example I demonstrates the coating of thoriumoxide spherical particles with a high density, isotropic pyrolyticcarbon coating utilizingpropaneas the coating and fluidizing gas.Examples II and III illustrate coating of thorium-uranium oxideparticles with a high density, isotropic pyrolytic carbon coatingemploying 1,3 butadiene and propylene, respectively, as the coating'andfluidizing gas. Preliminary irradiation data of thorium-uranium oxideparticles, which were coated with an initial thin layer of low density,porous carbon coating followed by a high density, isotropic outercoating are also given in Example 111.

Example I Fluidizing bed apparatus, consisting of a 1 ID graphitereaction chamber having a 36 included angle cone atthe bottom with awater-cooled injector, was used for preparing high density, isotropicpyrolytic carbon coated thorium oxide particles employing propane gas asthe coating and fluidizing gas at various temperatures and supply rates.

Several coating runs were made employing 28-gram charges of,460 microndiameter thorium oxide particles, respectively. The particles wereplaced in the reactor which was preheated to a deposition temperatureand fluidized with helium at a flow rate of 4.0 liters/minute until anequilibrium temperature within the range of 115'0-1300 C. was reached.Then the helium flow was switched to undiluted propane gas, which waspassed into the reactorto initiate the deposition of an isotropic, highdensity pyrolytic carbon coating. This deposition was carried out atflow rates between 1-4 cm. /min.-cm. and at propane partial pressuresbetween 190-760 mm. Hg. The coating phase was continued until about a-micron thick layer of pyrolytic carbon was deposited, which requiredabout eight minutes, and the deposition terminated.

A sample of the coated particles was removed from the reactor and thecoating densities determined. The coating densities were determined asfollows: the densities of the coated particles were measured using ahelium densitometer. The coatings were then removed by passing oxygenover the particles at 1000 C. and the evolved CO gas adsorbed onAscarite (NaOH on asbestos) and weighed which gives the weight of thecoatings. Assuming theoretical density (10.0 grams/cc.) for the thoriaparticles, the coating densities were then calculated.

Anisotropy measurements were made using graphite disks (7 diameter bythick) which were provided in each run along with the charge ofparticles. The technique was similar to that used by Bokros (Carbon,vol. 3, pp. 167-74, 1965) and was a modification of Bacons monochromaticpinhole technique (J. Applied Chem.

[London], vol. 6, pp. 477-81, November 1956).

. For comparison purposes a run was made using methaneat a depositiontemperature of 2000 C. which is illustrative of the prior method forobtaining high density, isotropic pyrolytic carbon coatings. The resultsare shown in Table I below.

TABLE I Coating Supply rate Partial Coating temp. (cm. /n1ln.- pressureThickness Denslt Run 0.) cm!) (mm. Hg) (11) (g./cm. )y Anisotropy 1,200 1. 0 190 66. 4 2. 05 Isotropic. 1, 250 1. 0 190 66. 4 2. 04 D0. 1,300 1. 0 190 58. 7 1. 96 Do. 2 1, 150 2. 0 380 66. 2 2. 04 D0. 1, 200 2.0 380 56. 4 2. 10 D0. 1 250 2. 0 380 61. 2 2.05 Do. 1, 300 2. 0 380 60.1 1. 96 Do. 3 1, 150 4. 0 760 65. 1 2.07 Do. 1, 200 4. 0 760 63. 5 2. 05Do. 1, 250 4. 0 760 47. 6 2. 02 Do. 1, 300 4. 0 760 1. 96 Do. 4 2 000 2.0 190 60 1. Do.

From the results it may be seen that the coatings deposited over therange of temperatures of 1150 -1300 C. were isotropic (i.e., having ananisotropy factor of about 1.0). Coatings deposited at 1150 C. andpropane supply rate of 1.0 cmfi/min-cm. indicated a slight anisotropiccharacter which is indicative of a general trend of increased anisotropyfactors as the temperature and propane supply rate are lowered.

Example 11 Several 28-gram batches of 250-300 micron diameter sphericalthorium-uranium oxide Particles were coated employing the same apparatusand techniques used in Example I, except undiluted 1,3 butadiene gas wassubstituted for propane. The supply rate of 1,3 Ibut'adiene was 4 cm./min.-cm. and a system pressure of one atmosphere pressure was employed.The equilibrium coating temperature, which varied between ll-1300 C.,was found to be about 40 degrees lower than the intial coatertemperature before deposition was initiated. Thi equilibriumtemperature, however, was easily maintained throughout the remainder ofthe coating run. The coat ings were all isotropic and had densities ofaround 2.0 g./cm. The results are shown in Table II below.

and the reactor temperature equilibrated to a temperature of 1200 C. fordeposition of the isotropic high density coating using undilutedpropylene gas as the coating and fiuidizing gas as hereinbeforedescribed. The coated particles were placed in the ORR Reactor andirradiated at a thermal neutron flux of 10 to a 3.6 at. percent uraniumburnup at 1400 C. Preliminary data of fission gas release rates, whichhave remained nearly constant during the irradiation, indicate thecoatings are of ex ceptional quality. The average fractional release for88 Kr is 3.5 X which is an extremely small release rate.

The above examples are merely illustrative and are not to be understoodas limiting the scope of th invention.

What is claimed is:

1. A method for preparing nuclear fuel particles having a high density,isotropic pyrolytic carbon coating thereon comprising the steps ofcontacting a bed of fluidized fuel particles with a coating g'asselected from the group consisting of propane, 1,3 butadiene andpropylene at a temperature in the range of 1100 to 1300 C.

2. The method of claim 1 wherein said coating oper- TABLE II CoatingSupply rate Partial Coating temp. (mul /min.- pressure Thickness Density0.) cm (mm. Hg) (g./cm. Anisotropy 1, 100 4.0 760 50.6 2.01 Isotropic.1, 150 4. 0 760 47. 5 1. 95 D0. 1, 200 4. 0 760 67. 2 1. 99 Do. 1, 2504. 0 760 65. 4 1. 99 Do. 1, 300 4. 0 760 58. 1 2. 07 Do.

Example 111 Several 28-gram batches of 250-300 micron diameter sphericalthorium-uranium oxide particles were coated employing the same apparatusand techniques used in Example 1, except undiluted propylene gas wassubstituted for propane. The supply rate of propylene was 4.0 cm./min.-cm. and a system pressure of one atmosphere was employed. Theequilibrium coating temperature, which varied from 1150 to 1300 C., wasfound to be about 40 degrees lower than the initial coater temperaturebefore deposition was initiated. This equilibrium temperature, however,was easily maintained throughout the remainder of the coating run. Theresults are shown ation is conducted at a flow rate of 1.0-4.0 cm./min.- cm.

3. The method of claim 1 wherein said coating operation is conducted at1200 C. with propane at a flow rate of 4 cmfi/min-cm. and at a pressureof 1 atmosphere.

4. The method of claim 1 wherein said coating operation is conducted at1200 C. with 1,3 butadiene at a flow rate of 4 cmF/min-cm? and at apressure of 1 atmosphere.

5. The method of claim 1 wherein said coating operation is conducted at1200 C. with propylene at a flow rate of 4 cm. /min.-cm. and at apressure of 1 atmosin Table III below. phere.

TABLE 111 Coating Supply rate Partial Coating 1 03 $1 1 (1.1510 1%? ifii2.7325 Anisotropy 1, 150 4. 760 78. 3 2. 02 Isotropic.

For irradiation studies a batch of 250-300 micron References Citeddiameter spherical thorium-uranium oxide particles were UNIT STATESPATENTS initially coated with a low density porous carbon coating1,893,236 1/1933 Iredeu 117 106 X and then with an isotropic, highdensity outer pyrolytic 05 2,414,625 1/1947 Abrams et al 117-46 X carboncoating. The particles were fluidized with helium 3,231,408 1/1966Huddle 117-100 X and brought to an equilibrium temperature of 1050 C.3,247,008 4/ 1966 Fmicle 117-100 X Then the helium flow was switched toundiluted acetylene 2; 2 5: 1 -t---l 17 501? e e e a gas, which waspassed into the reactor to m1t1ate the dep- 3,335,063 8/1967 Goeddel atal- 1 76 osition of the low density porous carbon coating, and thecoating conducted at a flow rate of 4 cm. /min.-cm. with a gas pressureof 760 torr for about 1.5 minutes until a first coating of about 40microns was deposited.

WILLIAM D. HART, Primary Examiner MATHEW R. PERRONE, Assistant ExaminerUS. Cl. X.R.

At this stage helium was substituted for the acetylene gas 117-100, 106;176-67, 91; 264-5

